Light-emitting diode is a semiconductor solid state device, including at least one p-n junction which is sandwiched between a p-type semiconductor layer and an n-type semiconductor layer. When a bias voltage is applied on the p-n junction, the holes from the p-type semiconductor layer and the electrons from the n-type semiconductor layer are combined to generate light. The region for generating light are generally called the active region.
The major characteristics of the light-emitting diodes are small size, high luminous efficiency, long life-time, quick response, high reliability, and good color rendering index. It has wide application, for example: in electronic equipment, automobile, signboard, or traffic lighting. Because the full-colors LEDs are presented to the public, LED has been replacing gradually the fluorescence lamp and the incandescent bulb along with other traditional illumination equipments.
Generally, the light-emitting diode collocates with the material of the wavelength conversion (example: phosphor) to generate the white light. The material of the wavelength conversion can excite the yellow light, green light, or red light when it is irradiated by the blue light emitted form the LED chip, and the blue light and the yellow light, or green light and red light are mixed to generate the white light. To assure the light generated from the LED chip can pass through the material of the wavelength conversion and mix to generate the light that desired, the material of the wavelength conversion has to cover the light emitting region from the LED chip completely. However, the light emitting direction is not specific, if the material of the wavelength conversion does not cover where the light emits from the LED chip completely and results in that partial light does not pass through the material of the wavelength conversion, such as the edge light, the wavelength conversion efficiency of the light is decreased. On the other hand, if the material of the wavelength conversion covers the LED chip completely, the wavelength conversion efficiency is enhanced, but it causes the heat dissipation problem.
It is not easy to spread the material of the wavelength conversion uniformly on the LED chip. When the thickness of the material of the wavelength conversion that covers the LED chip is not uniform, the thicker portion of the material of the wavelength conversion absorbs more light than the thinner portion of material of the wavelength conversion. The excited light is then different when the lights emitted to different directions facing different thickness of the material of the wavelength conversion.